Ethylene manufacture entails the use of pyrolysis furnaces (also known as steam crackers or ethylene furnaces) to thermally crack various gaseous and liquid petroleum feedstocks to ethylene and other useful products. Typical gaseous feeds to the pyrolysis furnaces include ethane, propane, butane and mixtures thereof. Typical liquid feedstocks to pyrolysis furnaces include naphtha, kerosene, gas oil, and other petroleum distillates.
The petroleum feedstocks are cracked in the tube reactors of the pyrolysis furnace at temperatures ranging from 700 to 1000.degree. C. Steam is generally injected in addition to the feed during the cracking reaction to control undesired reactions/processes, such as coke formation. In the typical operation of a pyrolysis furnace, the petroleum feedstocks and the steam are mixed and preheated through the convection section of the pyrolysis furnace.
Cracking of the petroleum feedstocks occurs in the radiant section of the pyrolysis furnace. The cracked product effluent from the radiant section is quenched through transfer line exchangers (TLXs) and oil and/or water quench towers, then fractionated and purified in the downstream processes to separate desired products. In general, ethylene is the major and the most desired of the products.
Metal alloys containing high nickel, iron and chromium are widely used in industry as the construction materials for pyrolysis furnace reactors because such alloys withstand the high temperature and extreme environmental operations. However, nickel and iron are also well-known catalysts for reactions leading to the formation of coke.
Coke deposits are the by-products of the cracking reactions. Even though the reactions leading to coke deposition are not significant relative to those producing the major desired products, the amount of the coke formed is enough to make the coke deposition a major limitation in the operation of pyrolysis furnaces. Fouling of the furnace reactors and TLXs (hereinafter collectively referred to as "pyrolysis furnaces") occurs because of the coke deposition. Coke deposition decreases the effective cross-sectional area of the process stream, which increases the pressure drop across the pyrolysis furnaces. The pressure buildup in the reactor adversely affects the yield of desired products, particularly ethylene. Additionally, because the coke formed on the inside of reactor tubes is a good thermal insulator, the buildup of coke requires a gradual increase in furnace firing to ensure enough heat transfer to maintain the desired conversion level. These higher temperatures accelerate reactor tube deterioration and shorten tube life.
Depending on the coke deposition rate, cracking operations must be periodically terminated or shut down for cleaning. Cleaning operations are carried out either mechanically or by passing steam and/or air through the coil to burn out the coke buildup. In addition to the periodic cleaning, crash shutdowns are sometimes required because of dangerous situations resulting from coke buildup in the pyrolysis furnaces. Run length, which is the operation time between the cleanings, may average from as little as one week to as long as four months depending in part upon the rate of fouling of the pyrolysis furnaces. Therefore, any process improvement or chemical treatment that could reduce coke deposition and thus increase run length would lead to higher production capacities, fewer days lost due to cleaning and lower maintenance costs.
Research has been carried out to understand the mechanisms under which coke formation occurs and to search for solutions to reduce or eliminate coke deposition. Coke can generally be classified into two categories: catalytic and non-catalytic coke. Dehydrogenation reactions catalyzed by metals, such as nickel and iron, are the origins of catalytic coke, while non-catalytic coke is the product of certain radical-type reactions. It is generally believed that the metal-catalyzed reactions play a more significant role in overall coke formation and deposition than the non-catalytic reactions. Thus, suppression of metal-catalyzed reactions would significantly lower overall coke formation and deposition.
Significant effort has been exerted over the past twenty years in developing coke inhibitors, i.e., chemical additives which suppress coke formation. Coke inhibitors work by passivating catalytically active metal sites through chemical bonding interactions, and/or forming a thin layer to physically isolate the metal sites from coke precursors in a process stream, and/or interfering with those radical reactions leading to coke formation by blocking active radical sites on surfaces.
Sulfur-containing species, such as sulfides (hydrogen sulfide (H.sub.2 S), dimethyl sulfide (DMS), dimethyl disulfide (DMDS)), mercaptans, and polysulfides, have been conventionally used in industrial practice to treat pyrolysis furnaces. Sulfur compounds have generally been used for CO formation control and coke formation inhibition. It is believed that sulfur forms a metal sulfide passivating layer on reactor metal surfaces and that this sulfide layer isolates gas phase coke precursors from active metal sites on surfaces, thereby resulting in coking reduction.
In addition to sulfur, phosphorus-based additives have also been reported to prevent coke formation in pyrolysis furnaces. Some of these phosphorus-containing additives contain sulfur bonded to phosphorus. Compounds having both sulfur and phosphorus discussed in the literature have sulfur to phosphorus atomic ratios of 4 or less.
The present inventors have discovered that more effective treatment procedures can be achieved by varying the ratio of sulfur to phosphorus. While both elements have been shown to be effective in commercial and lab units, their relative ratio has not been taken into consideration. Due to the wide variety of furnaces and their operating conditions, it is believed that certain circumstances will arise where the ratio might become critical to optimizing additive performance. No known literature or use has been reported where more sulfur, with respect to phosphorus, would be beneficial.
The use of sulfur compounds to control coke formation during the production of ethylene is shown in the prior art. For instance, U.S. Pat. No. 4,116,812 discloses a process of inhibiting fouling at elevated temperatures of 500.degree. F. to 1500.degree. F. by adding organo-sulflur compounds. In addition, U.S. Pat. No. 5,463,159 discloses a method of treating ethylene furnaces with hydrogen sulfide under a hydrogen and steam-containing environment to reduce CO and/or coke formation.
Likewise, phosphorus-containing formulations have been recognized as suppressants for coke formation in pyrolysis furnaces. The following patents disclose phosphorus compounds for inhibiting the formation of coke in pyrolysis furnaces. U.S. Pat. No. 3,531,394 discloses a method of reducing coke formation by providing for the presence of phosphorus and/or bismuth-containing compounds in the cracking zone. Elemental phosphorus is disclosed to be a coke preventative aid in refining units in U.S. Pat. No. 3,647,677. U.S. Pat. Nos. 4,024,050 and 4,024,051 disclose a method of inhibiting coke formation in petroleum refining processes using phosphate and phosphite esters, as well as inorganic phosphorus compounds. U.S. Pat. No. 4,105,540 teaches that phosphate and phosphite mono and diesters in small amounts function as antifoulant additives in ethylene furnaces. Certain phosphite esters, phosphate esters and thiophosphate esters are disclosed in U.S. Pat. No. 4,542,253 as being effective for reducing fouling in ethylene furnaces. U.S. Pat. No. 4,551,227 discloses a method of inhibiting coke formation in ethylene furnaces by treating the furnaces with a combination of tin- and phosphorus-containing compounds, or antimony- and phosphorus-containing compounds, or tin-, antimony- and phosphorus-containing compounds. U.S. Pat. No. 4,835,332 discloses a method of reducing fouling in ethylene furnaces by using triphenylphosphine. Phosphorothioates are disclosed in U.S. Pat. No. 5,354,450 as effective in the inhibition of coke formation in ethylene furnaces. Phosphoric triamides are disclosed as coke inhibitors for ethylene furnaces in U.S. Pat. No. 5,360,531.
Although sulfur and phosphorus compounds are known coke suppressant additives for pyrolysis furnaces, the use of a mixture of additives to provide a sulfur to phosphorus atomic ratio of 5 or greater is not disclosed in the prior art. The benefit of using an excessive amount of sulfur over phosphorus is not recognized in the prior art either. Accordingly, it is the object of this invention to provide an improved method for the inhibition of coke formation in pyrolysis furnaces using a combination of sulfur- and phosphorus-containing compounds having an atomic ratio of sulfur to phosphorus of at least 5.